Investments (Unrealized Losses and Fair Values) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	$ 8,266	$ 25,250
Less than 12 months, Unrealized losses	(291)	(191)
12 months or more, Fair values	2,818	2,061
12 months or more, Unrealized losses	(106)	(31)
U.S. Treasury notes [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	1,795	7,728
Less than 12 months, Unrealized losses	(58)	(70)
12 months or more, Fair values	831	195
12 months or more, Unrealized losses	(26)	(2)
Other government-related debt securities - U.S. [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	43	188
Less than 12 months, Unrealized losses	(1)	(1)
12 months or more, Fair values	66	34
12 months or more, Unrealized losses	(2)	(1)
Other government-related debt securities - Foreign and other [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	855	1,163
Less than 12 months, Unrealized losses	(38)	(9)
12 months or more, Fair values	74	115
12 months or more, Unrealized losses	(3)	(2)
Corporate debt securities - Financial [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	2,262	5,928
Less than 12 months, Unrealized losses	(74)	(28)
12 months or more, Fair values	397	462
12 months or more, Unrealized losses	(15)	(6)
Corporate debt securities - Industrial [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	1,970	5,760
Less than 12 months, Unrealized losses	(68)	(43)
12 months or more, Fair values	318	612
12 months or more, Unrealized losses	(12)	(9)
Corporate debt securities - Other [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	533	868
Less than 12 months, Unrealized losses	(21)	(4)
12 months or more, Fair values	13	117
12 months or more, Unrealized losses	0	(2)
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	673	1,838
Less than 12 months, Unrealized losses	(27)	(24)
12 months or more, Fair values	782	276
12 months or more, Unrealized losses	(35)	(6)
Other mortgage- and asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair values	135	1,777
Less than 12 months, Unrealized losses	(4)	(12)
12 months or more, Fair values	337	250
12 months or more, Unrealized losses	$ (13)	$ (3)